This invention relates to surgical apparatus and, in particular, to a miniature differential micrometer for a keratome.
A keratome is a surgical instrument for making thin slices of the cornea of the eye and resembles a tiny block plane having an overall length of about three centimeters. Unlike a block plane, the bottom of a keratome is not a single surface. An adjustable plate is located at the leading or front end of the keratome, followed by the cutting blade. Also unlike a block plane, the blade in a keratome oscillates from side to side at high speed to slice the corneal tissue.
In the normal cutting position of the keratome, the adjustable plate forms part of the bottom surface of the keratome and can be moved up and down, i.e. perpendicular to the bottom surface. (Any reference to direction is for the sake of description only. A keratome can be held and used for cutting in any orientation.) The adjustable plate is slightly trapezoidal in shape and one of the parallel edges of the plate is positioned adjacent the blade. The vertical spacing between the plate and the blade determines the depth or thickness of the cut.
The human eye is actually a compound Optical system including a fixed "lens," the cornea or outer surface, and a focusing lens. Severely myopic (nearsighted) patients often can have normal vision restored by thinning the cornea. In surgery on the cornea, the depth of cut is critical because the cornea of a human eye is typically 520 microns (0.020 inches) thick. In a procedure known as kerato-mileusis in-situ, the outer surface of the cornea, e.g. 150 microns, is removed in a first slice, a section 20-250 microns is removed in a second slice, and the first slice is re-attached to the eye. Obviously, the setting of the blade is extremely critical for each cut. The depth of cut must be known precisely and accurately.
In the past, keratomes have been provided with an adjustment screw having a fine thread, enabling small changes to be made in the depth of cut. The problem is knowing how deep the cut will actually be. Typically, a gauge is provided for measuring the displacement of the plate from the blade. The problem with a gauge is that it cannot withstand sterilization. Since the surgical field and all instruments must be sterile, adjusting the blade means placing the keratome in a non sterile gauge, setting the depth of cut by touching the plate with part of the gauge, and then sterilizing the keratome.
Thus, it is desired to have a keratome with an integral, calibrated adjustment. Micrometer adjustments of the prior art rely on a fine threaded screw but even this is not very accurate. For accurate correction of myopia, ophthalmic surgeons want an adjustment accurate to .+-.(2-5) microns. While micrometers capable of this accuracy are known, they dwarf the keratome in size and are unsuited to being incorporated into a keratome.
Berkman et al. U.S. Pat. No. 4,750,489--discloses a surgical knife for use in a procedure known as radial keratotomy in which shallow, radial slits are made in, but not through, the cornea. The knife includes a micrometer and electronics within the knife are used for "eliminating reliance on the accuracy of the adjusting screw." In other words, the micrometer is not sufficiently accurate for surgery on the eye. A calibration system is disclosed for sensing the zero or "datum" point on the extension of the blade from the end of knife. In surgery, the electronics in the knife indicate the amount the blade is extended.
The most accurate micrometers known are differential micrometers. These devices have a single screw with threads of two different pitches. The movement of the contact point in the micrometer is determined by the difference in pitch of the threads. For example, if one thread were twenty turns per inch (0.05 inches per turn) and the other thread were twenty five turns per inch (0.04 inches per turn), then one turn of the screw would produce a movement of 0.01 inches of the contact point. Differential micrometers of the prior art are typically four-six inches long, far too large for a keratome.
Differential micrometers cannot simply be scaled down in size. Everything that is made smaller is also made weaker. For example, tension nuts, needed to avoid backlash, break during use when a differential micrometer is made small enough to be incorporated into a keratome. (Backlash is movement of the contact without rotation of the barrel or no movement of the contact with some rotation of the barrel.)
Even if a small differential micrometer were available, the problems of calibration and sterilization remain. Surgeons want a device with a dial, easily read, that can be adjusted in the operating room, knowing that the cut will be the thickness indicated. In this way, two consecutive cuts of different thickness can be made without contaminating the keratome between cuts with a calibration gauge.
The dial should be easily read and provide the surgeon with a feel for how much the plate has been adjusted. Typically, the barrel of a micrometer moves as it is rotated to reveal more or less of a fixed scale. If the pitch of the thread on which the barrel turns is made arbitrarily fine, then the barrel moves little. Thus, there are contrary demands on the pitch of the threads: a larger pitch causes the barrel to move a greater distance while a smaller pitch increases the resolution of the micrometer.
In view of the foregoing it is therefore an object of the invention to provide a differential micrometer fitting on one end of a keratome.
Another object of the invention is to provide a keratome which is calibrated during manufacture and does not require re-calibration thereafter.
A further object of the invention is to provide a keratome which can cut with a resolution of one micron and an accuracy of .+-.(2-5) microns.
Another object of the invention is to provide a keratome which can be sterilized and used without further calibration.
A further object of the invention is to provide a miniature differential micrometer.
Another object of the invention is to provide a miniature differential micrometer having a relatively large longitudinal movement of the barrel in proportion to the movement of the contact point.